fix: wire optional AI SDK into plugin installation

[Pigbibi]

Purpose

The plugin imports ai_gateway_client but did not declare a way to install it. Add an explicit ai extra pinned to the existing standard-library SDK from AAB #158. Default runtime dependencies and AI enablement remain unchanged.

Scope

Five files: package metadata/lock, existing AI tests, existing CI, README. No plugin source, model/auth API, workflow permissions, business switch or runtime configuration changes. CI explicitly installs test,ai and tests the real SDK, mocking only HTTP/OIDC network boundaries.

Validation

• Fresh base: actual installed-SDK regressions RED, 3 failed (ModuleNotFoundError).
• Locked non-editable install with ai extra: successful; SDK Git commit matches declared pin.
• Network-denied AI tests: 56 passed; full suite: 247 passed.
• Fresh default installation: SDK absent and AI disabled.
• Offline lock/dependency checks, Ruff and sdist/wheel build passed.
• Existing lock drift: base lock had Ruff 0.16.4 while pyproject already required 0.16.6. Lock now matches that existing requirement; all other existing dependency identities unchanged.

No actual model, paid API, provider, deployment, publishing or feedback call. Production consumer adoption, source authorization and remote validation remain separate.